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Defeating the Hydrilla Monster

According to experts, the water monster arrived in the state courtesy of residents’ fish aquariums.

Used as an aquarium plant, hydrilla was intentionally planted in Florida canals in the 1950s by aquarium plant dealers.

Since escaping and invading water bodies throughout Florida, the plant, which is native to Southeast Asia, has filled Florida lakes and choked out other native plants. According to University of Florida plant experts, controlling hydrilla costs more money than controlling any other invasive plant in the state.

Most challenging?

Solutions to the hydrilla problem often trigger other problems.

At the June 21 meeting of the Park Place Community Development District (CDD), which serves Highland Park, Mandolin and Windsor Townhomes, the treatment of hydrilla infestation in Lake Dagny became a point of contention between a few Highland Park homeowners and their CDD board. In recent years, as Lake Dagny became nearly entirely choked by the plant, the district introduced sterile carp to eat the lake free. Now free of hydrilla, the lake’s formerly clear water looks like a standard Florida lake, prompting two homeowners to question whether the district’s treatment of hydrilla has led to poorer water quality and a lake environment entirely devoid of all plant life.

Take a summer walk along lakes or ponds in Northwest Hillsborough and you’ll eventually spot it sitting in clumps at the water’s surface. Hydrilla is a long-tube like plant whose stalk is covered by hundreds of small leaves. From a distance, it looks a bit like wet, submerged, bright green branches of a fir tree.

While some insist that moderate hydrilla infestations of roughly 30 to 40 percent of a pond help aquatic life thrive, hydrilla’s voracious growth rate makes it nearly impossible to contain at this level. Left unchecked, hydrilla will entirely choke out any lake with depths of 25 feet or less.

“It came in a few years back,” said Westchase CDD Field Manager Doug Mays, who said hydrilla wasn’t present in Northwest Hillsborough ponds when he started his Westchase tenure more than a decade ago. “It’s become quite prevalent in the last eight years. Now it’s starting to become part of a problem.”

The damage hydrilla causes in Florida’s canals, lakes and ponds is so significant, the plant is now on the state’s banned list. It can’t be legally sold, brought in, cultivated or transported in the state.

Hydrilla can grow in nutrient rich and nutrient poor lakes. Its long stems can grow up to 25 feet in length, choking water flow, out-competing other lake life and actually limiting any recreational use of large lakes in which it appears. Its large mats of growth entangle boat propellers and have tragically even caused some entrapped swimmers to drown.

“When hydrilla reaches the surface, it forms dense mats that block the penetration of light and oxygen into the water column,” wrote UF experts Lyn A. Gettys and Stephen F. Enloe in their article titled "Hydrilla: Florida's Worst Submerged Weed."

Hydrilla grows prolifically. While often touted as being able to grow an inch per day, that description represents a startling underestimate. In one 2012 study, two scientists measured a five-inch hydrilla cutting as it branched and grew over five weeks. Collectively, the original cutting represented new growth of 191 inches per day.

The UF researchers, Gettys and Enloe, added that its quick growth enables hydrilla to outcompete and displace other aquatic plants. This, in turn, creates a limited habitat for fish, turtles and other aquatic wildlife that depend on a more diverse group of aquatic plants for breeding and survival.

They added, “Dense plant growth and surface mats also trap heat, which raises the temperature of surface water and depletes dissolved oxygen.”

Unchecked hydrilla growth not only affects fish populations but also the birds that feed on them. In recent years, hydrilla has been found to host blue-green algae (cyanobacteria). It's a neurotoxin that, once consumed by fish, is passed on to birds that eat the fish. The bacteria then attack the nervous system of birds. Scientists have attributed deaths among ducks and geese and even eagles and great horned owls to the hydrilla-hosted algae.

The bacteria can even pose a danger to small pets. The threat from pond alligators aside, pets should never be permitted to wade in water containing hydrilla. The slimy coating of the bacteria from hydrilla, which tends to start growing in the shallows of pond banks, can cling to pets’ coats and sicken them during self-cleaning.

The large aquatic mats created by hydrilla also create ideal breeding grounds for mosquitoes. Mosquito larvae in the dense matted growth are protected from being eaten by fish.

The UF scientists also said that hydrilla-choked ponds and canals also dramatically slow the flow of large amounts of water produced by tropical storms and hurricanes, an essential function of the retention pond system in the state’s suburbs.

All the research WOW found on hydrilla pointed to the same conclusion: any hydrilla in a body of water represented a potentially long-term and expensive problem unless caught and killed early.

Yet controlling hydrilla even early on is a challenging and pricey endeavor. Non-chemical approaches such as culling or cutting hydrilla out of ponds generally leads to further infestation. Shoot fragments that are left behind readily re-sprout in the lake.

So lake maintenance experts have looked to the animal and insect world for solutions.

A handful of insects are known to eat or infest hydrilla, but their use has yet to prove an effective or economically beneficial control mechanism.

In the 1980s, scientists ultimately developed a sterile version of carp, called triploid grass carp. Voracious hydrilla eaters, they’ll strip a water body clean of the plant. The carp were ultimately embraced by the Park Place CDD when the chemical treatment of hydrilla on Lake Dagny proved unpopular with some homeowners along the lake and compelled the district to find a different solution.

The carp run about $10 per fish. According to Westchase CDD Field Manager Doug Mays, when purchased, the carp are about nine to ten inches long, making them initially ideal prey for water fowl. The carp that survive, however, soon impressively outweigh the birds. “When they die, they’re almost three or three and a half feet long,” said Mays, who emphasized that the fish are only plant eaters. In recent years the Westchase district staff was called out to address two enormous carp floating in a lake in The Shires. “They stink up a storm when they’re dead,” said Mays.

Use of the carp, however, is not without downsides. Their diet often includes other plant species that are good and desirable in a healthy lake. Stock a lake with too many carp and they’ll strip it clean of all plants, affecting the ability of fish and other aquatic animals to breed, reproduce and hide from predators. And while many of the carp become prey for aquatic birds and alligators, the surviving fish can live up to 15 years and are elusive to capture.

Given the damage they can do, the state requires a permit before a lake or pond can be stocked with even sterile carp, which can’t reproduce.

“About five years ago we pulled a permit for some carp. I want to say we bought close to 100 carp that time,” said Mays, who added they were distributed across a lot of Westchase lakes. “We’ve been working on pulling another permit for some carp now.”

Mays said that while they have the permit for 100 more of the fish, it requires the district to erect some barriers to keep the carp out of bodies of water where their behavior could impact other fish.

The carp, Mays said, are used in conjunction with chemical treatments.

Herbicides, while effective, are quite costly. Late in 2016, A & B Aquatics, the company that maintains Westchase ponds, pointed to the rising cost of chemicals used to control hydrilla as the primary reason behind their request for a $10,000 increase in its annual contract.

Chemical treatments also pose challenges. In the 1980s and 1990s, the most commonly used herbicide for hydrilla was Fluridone. In the late 1990s, however, much of the hydrilla in Central Florida had developed resistance to the widely used chemical. Subsequently, other more effective, quicker acting herbicides have been developed, but experts cautioned that pond maintenance companies should rotate the herbicides they use to avoid the hydrilla becoming resistant again.

In ancient Greek mythology, the Hydra is a monster with multiple heads. Lop off one and two more grow in its place. It took the mighty and cunning Hercules to address the problem as one of his twelve labors.

And while Greek heroes may be hard to come by of late, the hydrilla lake monster can be tamed with patience, balance, an open mind and a very open pocketbook.

By Chris Barrett, Publisher


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